The present invention generally relates to an apparatus for detecting an instantaneous (momentary) reactive current in a three-phase load and an apparatus for compensating a reactive power by means of a converter in which the instantaneous reactive current detecting apparatus is made use of. More particularly, the invention is concerned with an apparatus for compensating an instantaneous reactive power in a three-phase power supply source by means of a circuit including a current-source type converter connected in parallel with the three-phase load without exerting influence to the output voltage of the converter circuit.
A local electric system is extensively utilized in which a converter load circuit including a current-source type converter and a general purpose load circuit including conventional three-phase loads are connected in parallel with a three-phase power supply source. The converter load circuit mentioned above includes DC loads connected thereto through the current-source type converter or AC loads such as AC motors connected by way of an inverter circuit. In such electrical systems, many of the general purpose load circuits include various inductive loads which provide a cause for lowering the power factor, giving rise to generation of a corresponding reactive power which necessary imposes a burden on the three-phase power supply source. Under the circumstances, it would be very desirable if the reactive power could be compensated by the converter load circuit, since the burden imposed on the three-phase load due to the reactive power then could be mitigated, making it possible to reduce the capacity of the power supply equipment for a given effective power supply. In order to allow the reactive power to be compensated by means of the current-source type converter, it will be effective to compensate the reactive current corresponding to the reactive power through the current control by the converter circuit. To this end, it is necessary to detect the instantaneous reactive urrent in the general purpose load circuit and compensate the instantaneous reactive current by means of the converter circuit.
As the instantaneous reactive current detecting apparatus known heretofore, there can be mentioned a system disclosed in an article entitled "General Analysis of Instantaneous Reactive Power and Application" contained in "Periodical of the Institute of Electrical Engineers of Japan", Vol. 103, No. 7, p.p. 483-490. This known detecting apparatus is of a much complicated circuit configuration including a three-phase/two-phase conversion circuit. Certainly, this known detecting apparatus is advantageous in that the instantaneous reactive current can be detected even when the source voltage suffers distortion in the wave form. In general, however, the waveform of the source voltage is rarely susceptible to distortion, and any significant error would not be involved even on the assumption that the source voltage is of a three-phase sinusoidal wave form. Accordingly, there exists a demand for the instantaneous reactive current detecting apparatus of much simplified circuit arrangement.
As another means for detecting and determining the instantaneous reactive power, there is known a method according to which each phase current of the power supply source is multiplied with the value of the associated phase voltage shifted by 90.degree., wherein the products obtained for three phases, respectively, are added together. Reference may be made to Japanese Patent Application Laid-Open No. 125035/1980 (JP-A-55-125035). However, no disclosure is contained in the above publication concerning the detection of the instantaneous reactive current.
As the reactive power compensating device, capacitors have heretofore been employed in many applications. In recent years, there arises a tendency to use active filters.
With the capacitor, only the reactive power due to lag can be compensated, and the reactive current which can be supplied is restricted only to the current of sinusoidal wave. On the other hand, with the active filter system in which GTO and/or transistors are employed, the reactive power due to the leading current can be compensated and the reactive current of the waveform other than the sinusoidal wave can also be supplied.
If the reactive current having the waveform other than the sinusoidal wave and containing harmonic components could be compensated, the harmonic current components of the power supply source could also be decreased, whereby the source waveform will be further improved when compared with the case where the capacitor is used as the reactive power compensating device. To this end, the harmonic components of the reactive current must be detected. Detection of the reactive current without time delay has, however, encountered difficulty with the hitherto known instantaneous reactive current detecting apparatus because of much complicated circuit configuration including a three-phase/two-phase conversion circuit and the like.
As another instantaneous reactive power compensating apparatus known heretofore, there can be mentioned a reactive power control in which a voltagesource type converter is employed, as disclosed in an article entitled "Operation Analysis and Designing of Instantaneous Reactive Power Compensating Apparatus Using Voltage-Source Type PWM Converter" contained in "Periodical of the Institute f Electrical Engineers of Japan", Vol. B106, No. 4, p.p. 323-330. However, neither teachings nor suggestion has ever been made as to the use of the current-source type converter and the control of the output current thereof for the purpose of compensating the instantaneous reactive power up to the possible maximum capacity of the current converter.